Question: What is the value of $x^2+y^2-z^2+2xy$ when $x=-3$, $y=5$, and $z=-4$?
Solution: We substitute the values of $x$, $y$, and $z$ to get $$(-3)^2+(5)^2-(-4)^2+2(-3)(5)=9+25-16-30=34-46=\boxed{-12}.$$